Imaging system

ABSTRACT

A photoelectrophoretic imaging system wherein formation of an insulating surface on an electrode is accomplished by particle migration of a particulate resinous material onto the electrode prior to imaging. The resinous material is preferably suspended in the imaging suspension.

United States Patent 1191 Tulagin et al.

[ Nov. 27, 1973 IMAGING SYSTEM [75] Inventors: Vsevolod Tulagin, Rochester;

Leonard M. Carreira, Penfield, both of N.Y.

[73] Assignee: Xerox Corporation, Rochester, N.Y.-

[22] Filed: Oct. 31, 1969 [21] Appl. No.: 872,851

Tulagin et a1 204/ 181 Clark 204/181 Primary Examiner-George F. Lesmes Assistant Examiner-M. B. Wittenberg Attorney-David C. Petre, Richard A. Tomlin and James J. Ralabate [57] ABSTRACT A photoelectrophoretic imaging system wherein formation of an insulating surface on an electrode is accomplished by particle migration of a particulate resinous material onto the electrode prior to imaging. The resinous material is preferably suspended in the imaging suspension.

7 Claims, 5 Drawing Figures [52] US. Cl 96/1.4, 96/l.2, 96/1.3, 1l7/17.5 LE [51] Int. Cl. 603g 13/14, 603g 13/16 [58] Field of Search 96/1, 1.2, 1.3, 1.4; 252/501; 204/181; 117/17.5

[56] References Cited UNITED STATES PATENTS 2,940,847 6/1960 Kaprelian 96/1 ,8 BXXI Q. /9

PAIENIE0nnv21 nan FIG. 2a

FIG. 20

INVENTORS VSEVOLOD TULAGIN LEONARD M. CARREIRA ATTORNEY 1 IMAGING SYSTEM BACKGROUND OF THE INVENTION This invention relates in general to imaging systems. More specifically, the invention concerns a photoelectrophoretic imaging system utilizing reflex imaging.

There has been recently developed an electrophoretic imaging system capable of producing color images which utilizes electrically photosensitive particles. This process is described in detail and claimed in U. S. Pat. Nos. 3,384,566 to H. E. Clark, 3,384,565 to V. Tulagin and L. Carreira, and 3,383,993 to Shu-Hsiung Yeh. In such an imaging system, variously colored light absorbing particles are suspended in a non-conducting liquid carrier. The suspension is placed between electrodes, one of which is generally conductive, called the injecting electrode and the other of which is generally insulating and called the blocking electrode.

One of these electrodes ispreferably at least partially transparent to activating electromagnetic radiation. The suspension is subjected to a potential difference between the electrodes across the suspension and exposed to an image through the transparent electrode. As these steps are completed, selective particle migration takes placein image configuration, providing a visible image at one or both of the electrodes. An essential component of the system is the suspended particles which must be electrically photosensitiveand which apparently undergo a net change in charge polarity upon exposure to activating electromagnetic radiation when brought into interaction range of one of the electrodes. Ina monochromatic system, a single colored image equivalent to conventional black-and-white photographyis produced. In a polychromatic system, images may be produced in natural color by using mixtures of particles of two or more different colors which are each sensitive to light of a specific wavelength or narrow range of wavelengths. I

In a polychromatic system photosensitive particles are caused to initially migrate to the transparent conductive injecting electrode. Conventionally cyan particles responsivemainly to red light, magenta particles responsive mainly to green light and yellow particles responsive mainly to blue light are dispersed in an insu-' lating liquid. When, for example, the particles are illuminated by red light, cyan particles migrate away from the electrode leaving behind the magenta and yellow particles which combined appear r'ed. Where particles are illuminated by white light all particles are removed and where there is no light all particles remain providing white and black respectively on the surface of the injecting electrode. This image may be removed, for example, by adhesive transfer, electrostatically, or other suitable means. Further the injecting electrode could be made up of a transparent conductive plastic such as cellophane or a thin layer of a plastic could be placed over the electrode providing a substrate to which the image could be readily permanently affixed by, for example, heating or by laminating a sheet over the image.

This system, using preferably a transparent conductive injecting electrode, a substantially insulating blocking electrode and photosensitive particles dispersed in an insulating carrier liquid, between the electrodes has been found to becapable of producing excellent images.One'majordisadvantage in the prior art systemsis the accumulation-of charge on the surface of the blocking electrode which interferes with particle migration. Charge build up is a major problem for continuous cyclic imaging. Application Ser. No. 626,917, filed Mar. 28, 1967 by R. N. Ciccarelli describes some methods for eliminating the charge build up by application of neutralizing charges for example. These process require additional power.

Further the imaging suspension is conventionally a material which may act as a solvent such as kerosene. These materials soften and otherwise chemically affect the blocking electrode surface under continued use which results in changed electrical characteristics. Eventually the blocking electrode must be replaced.

Continued cyclic imaging also causes abrasion of the blocking electrode by the suspended pigment particles which again affects final image quality.

SUMMARY OF THE INVENTION It is, therefore, an object of this invention to provide a photoelectrophoretic imaging system which overcomes the above noted disadvantages.

It is another object of this invention to provide a continuous cyclic photoelectrophoretic imaging system which does not require elimination of charge accumulation.

It is another object of this invention to provide a photoelectrophoretic imaging system which does not require a relatively abrasion and solvent resistant blocking electrode.

The above objects and others are accomplished in accordance with this invention by providing a photoelectrophoretic imaging system in which an imaging suspension of electrically photosensitive pigment particles and a particulate resinous insulating material in an insulating carrier liquid is placed in an electroded system. The pigments and resin are selected such that when placed in a field the pigments are drawn toward one electrode and the resin toward an opposing electrode forming an insulating layer thereon. Preferably the pigments are drawn toward a transparent conductive drum or plate The imaging suspension is then exposed to imagewise electromagnetic radiation causing the initially insulating pigments to exchange charge with the electrode and be repelled by it. The repelled particles migrate to the opposing electrode on which the resin layer has been formed. The pigments remaining form a substractive positive image which may be of one color or maybe polychromatic as described above.

The particulate resin layer prevents the pigments from exchanging charge and returning to the conductive layer. Since the resin coated electrode is less capable of charge exchange it may be referred to as the blocking electrode, the conductive electrode is referred to as the injecting electrode since it is more capable of charge injection or exchange.

The resin layer and the unwanted pigment particles on the blocking electrode are then removed by any suitable means. They may be scraped or brushed off of the blocking electrode or washed off by a spray of liquid. The cleaning of the blocking electrode may be aided by application of potential opposite to that used during imaging to eliminate electrostatic attraction. The blocking electrode is then ready for reuse. Since the insulating layer is replaced each cycle no degradation, charge build up or wear can occur.

The resin may be provided other than in the imaging suspension, for example, by deposition from a separate suspension. However, it simplifies the system to use the imaging suspension as the resin carrier. This invention makes use of the fact that particles suspended in insulating liquids naturally acquire a charge and may be attracted to an electrode. It is necessary therefore, to select particles of resin which in general are attracted to the blocking electrode and to have pigments which are attracted to the injecting electrode, that is, that the resin and pigment components take on opposite charges when dispersed in an insulating liquid.

The resinous or plastic material may be thermoplastic or thermosetting and should be insulating and relatively insoluble in the liquid carrier used. Typical thermoplastic or thermosetting compounds include, vinyl polymers, polystyrenes, polyethylenes, polypropylenes, polyimides, polyamides, polyesters, polycarbonates, phenoxys, fluorocarbons and mixtures and copolymers thereof where applicable. Polyethylene is preferred because of its good electrical properties and it suspends well in hydrocarbon solvents.

The imaging suspension in which the resin is dispersed may comprise any suitable electrically photosensitive particles dispersed in a carrier liquid and may be of two or more colors. Typical electrically photosensitive particles and carrier liquids are disclosed in U.S. Pat. No. 3,384,488 issued May 21, 1968 to V. Tulagin et al., U.S. Pat. No. 3,447,922 issued June 3, 1969 to L. Weinberger and U.S. Pat. No. 3,357,989 issued Dec. 12, 1967 to J. F. Byme et al., the disclosures of which are incorporated herein by reference. The x-form of phthalocyanine is preferred for monochrome imaging because of its high sensitivity. For polychrome imaging a mixture of about equal parts of phthalocyanine, Watchung Red B and a yellow pigment N-2-pyridyl- 8,13-dioxodinaphtho-(2,I-b;2',3'-d)-furan-6- carboxamide prepared as shown in U.S. Pat. No. 3,447- ,922 is preferred because of its high sensitivity and excellent color separation quality.

It is preferred that the particles of resin have a size of about 1 to about microns.

The imaging suspension may initially be coated on the injecting electrode or the blocking electrode. Typical coating methods include roller application, dip coating, spraying, or brushing.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages of this improved method of photoelectrophoretic imaging will become apparent upon consideration of the detailed disclosure of the invention especially when taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a side sectional view of a simple exemplary photoelectrophoretic imaging system in accordance with this invention.

FIGS. 2a through 2d show diagramatically the hypothesized particle movement within the imaging suspension. The dimensions and particle distributions are distorted for clarity.

Referring now to FIG. 1 there is seen a transparent electrode generally designated 1 which in this exemplary instance is made up of a layer of optically transparent glass 3 overcoated with a thin optically transparent layer of tin oxide, commerically available under the name NESA glass. This electrode is referred to as the injecting electrode".

Coated on the surface of electrode 1 is a thin layer of an imaging suspension generally designated 7 which comprises electrically photosensitive particles and resin particles 8 dispersed in an insulating carrier liquid 9.

A second electrode 10 in this exemplary instance comprises a conductive aluminum roller. The conductive center of roller 10 and electrode 1 are connected to a source of d.c. potential 11 and ground.

In operation imagewise radiation 16 is used to expose imaging suspension 7. With power source 11 activated roller 10 is caused to roll across imaging suspension 7.

On completion of roller traverse a positive image is found adhering to the surface of electrode 1. A negative image is formed on the blocking electrode. This image may be transferred and used if desired. Conventionally, however, the image formed on the surface of electrode 1 is transferred to a transfer member and fixed thereon. Blocking electrode 10 is then caused to rotate, doctor blade 12 being used to scrape off the unwanted plastic and pigment particles. To aid transfer the sign of the potential on electrode 10 may be reversed. Although FIG. 1 shows the blocking electrode being held at a negative potential with respect to the injecting electrode the polarities may be reversed depending on the electrical characteristics of the pigments and the plastic particles in the imaging suspension.

Referring now to FIG. 2a the randomly dispersed particles of pigment l9 and resin 18 are shown responding to field applied between electrodes 20 and 21 forming the layered structure of FIG. 2b. In actual practice there may be more than a single layer of pigment or resin formed.

FIG. 2c illustrates the movement of the pigment particles in response to activating electromagnetic radiation and field. The particles which are exposed to wavelengths to which they respond exchange charge with transparent conductive electrode 21 and are repelled by it being attracted toward electrode 20. Since particles 19 are prevented from rapidly exchanging charge with electrode 20 by layer of insulating resin particles 18 a positive image remains on electrode 21 and a negative image is formed on the surface of electrode 20. Either image may be used, however, for multicolor substractive imaging and to maximize the benefits of this invention the positive image on the surface of electrode 21 is transferred to another member and fixed thereon and the resin layer and negative image are scraped off.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following examples further specifically illustrate the'improved photoelectrophoretic imaging system of this invention. Parts and percentages are by weight unless otherwise indicated. The examples below are intended to illustrate various preferred embodiments of the present invention. All of the following examples are carried out in apparatus of the general type illustrated in FIG. 1.

The conductive surface of the injecting electrode is connected to ground and in series with a potential source, a switch, and a conductive roller. The roller is approximately 6 inches in diameter and is moved across the plate surface at about 2 inches per second. The plate injecting electrode employed is roughly 12 inches square and is exposed with a light intensity of about 8,000 foot candles as measured on the uncoated injecting electrode surface. Unless otherwise indicated,

a total of about 7 percent by weight of the indicated pigments in each example are suspended in Sohio Odorless Solvent 3440, a kerosene fraction available from Standard Oil of Ohio, and the magnitude of the applied potential is 2,500 volts, the roller being negative relative to the injecting electrode. Exposure is made with a 3,200K. lamp through a full color Kodachrome transparency. A complete imaging cycle including cleaning takes about 6 seconds.

EXAMPLE I An imaging suspension is prepared containing about 0.75 grams of magenta pigment, Watchung Red B, a barium salt of l-(4'-methyl-5'-chloroazobenzene-2'- sulfonic acid)-2hydroxy-3-naphthoic acid, C.l. No. 15865; about 1.2 grams of a yellow pigment, N-2

carboxamide, and about 1.8 grams of a cyan pigment, Monolite Fast Blue G.S., the alpha form of metal-free phthalocyanine, C]. No. 74100 in 50 milliliters of Sohio Odorless Solvent 3440. The pigments are finely divided averaging about 0.5 microns in diameter. Approximately 5 parts of particulate polyethylene microthene FN-SOO having a particle size of less than microns is then added to the suspension. On completion of roller traverse a positive full color image is formed on the surface of the injecting electrode. The image is transferred to paper electrostatically. The conductive roller is then scraped and cleaned by a hard rubber squeegee. The above process is continuously repeated, the images being of consistent quality.

EXAMPLE ll PRIOR ART The experiment of Example 1 is repeated except that the conductive roller is covered with a Tedlar (polyvinylfluoride) sheet and no polyethylene is added to the imaging suspension. After approximately three images are formed image quality is found to deterioriate due to charge build up and wear on the Tedlar blocking layer surface.

What is claimed is:

1. A method of forming images from electrically photosensitive particles comprising the steps of:

a. forming a layer of an imaging suspension compris ing electrically photosensitive particles and insulating particles dispersed in a substantially insulating carrier liquid between a first transparent conductive electrode and a second electrode, said electrically photosensitive particles being attractable to a polarity of the opposite sign;

b. applying an electrical field across said suspension of a polarity selected to attract the electrically photosensitive particles to said first electrode and said insulating particles to said second electrode until a uniform layer of insulating particles is formed on said second electrode only;

. exposing said imaging suspension to a pattern of electromagnetic radiation of wavelengths to which at least a portion of said electrically photosensitive particles are responsive through said first electrode while maintaining said electrical field across said imaging suspension until an image is formed from said electrically photosensitive particles.

2. The method of claim 1 wherein a monochromatic image is formed.

3. The method of claim 1 wherein electrically photosensitive particles of more than one color are used, said particles of a first color having a spectral response curve which does not substantially overlap the spectral response curve for particles of a different color and said electromagnetic radiation contains wavelengths of radiation to which more than one color particle responds. g A

4. The method of claim 3 wherein said electrically photosensitive particles comprise yellow particles responsive mainly to blue light, magenta particles responsive mainly to green light and cyan particles responsive mainly to red light.

5. The method of claim 1 including in addition the steps of:

f. cleaning said second electrode; g. transferring said image from said transparent conductive electrode; and h. repeating steps a through e at least one additional time.

I 6. The method of claim 1 wherein said insulating material comprises finely divided particles of a resin.

7. The method of claim 1 wherein said resin comprises polyethylene. 

2. The method of claim 1 wherein a monochromatic image is formed.
 3. The method of claim 1 wherein electrically photosensitive particles of more than one color are used, said particles of a first color having a spectral response curve which does not substantially overlap the spectral response curve for particles of a different color and said electromagnetic radiation contains wavelengths of radiation to which more than one color particle responds.
 4. The method of claim 3 wherein said electrically photosensitive particles comprise yellow particles responsive mainly to blue light, magenta particles responsive mainly to green light and cyan particles responsive mainly to red light.
 5. The method of claim 1 including in addition the steps of: f. cleaning said second electrode; g. transferring said image from said transparent conductive electrode; and h. repeating steps a through e at least one additional time.
 6. The method of claim 1 wherein said insulating material comprises finely divided particles of a resin.
 7. The method of claim 1 wherein said resin comprises polyethylene. 